Topic 3.9 — this Keyword

Goal: use this to refer to the current object, especially to resolve shadowing and to access instance variables/methods from inside the class.

The big idea

Inside an instance method or constructor, this is a reference to the current object (the object the method is running on).

You use this most often to fix shadowing and make it clear which variable is the field.

If a parameter has the same name as an instance variable, the parameter “hides” the instance variable.

public class Student {
  private int grade;

  public void setGrade(int grade) {
    grade = grade;  // ❌ sets the parameter to itself
  }
}

The left grade is the parameter, not the field.

Use this.grade to mean “the field named grade.”

public void setGrade(int grade) {
  this.grade = grade;  // ✅ field = parameter
}

Same idea: it helps you initialize fields clearly.

public class Student {
  private String name;

  public Student(String name) {
    this.name = name;
  }
}

You can call methods like this.doThing(), but most of the time it’s optional.

public void reset() {
  this.setGrade(0); // works
  // setGrade(0);   // also works (same object)
}

Static methods don’t run on a specific object, so there is no “current object.”

public static void demo() {
  // System.out.println(this); // ❌ not allowed
}